The present invention relates to an image forming apparatus and a method of producing a threshold value matrix. More specifically, the present invention relates to an image forming apparatus for converting an image received as an original image to a halftone image to form an image (referred to as halftone conversion).
Conventionally, when a multi gradation image is converted to a halftone image, a one-on-one comparison method has been used. In the one-on-one comparison method, first, an original image is recorded, and the original image is covered with a threshold value matrix in a tile pattern. Then, a pixel number is compared. When a comparison value is greater than a threshold value, a dot is turned on (an on dot or a black dot). When the comparison value is smaller than the threshold value, the dot is turned off (an off dot or a white dot). Accordingly, in the one-on-one comparison method, it is possible to obtain a binary image.
In the threshold value matrix, in general, a threshold value is arranged such that a dot pattern having a circle shape or a line shape is repeatedly formed with a specific periodic cycle and a specific angle. When a printing apparatus as an image forming apparatus possesses a high resolution, it is possible to sufficiently realize the gradations of the original image with an area ratio of one halftone.
However, when the printing apparatus is limited to possess a low resolution, it is possible to realize only limited gradations with one halftone. For example, when the printing apparatus having the output resolution of 600 DPI (600 dots per one inch) is used to form the halftones of 141 lines (141 periodic cycles per on inch), it is possible to realize 19 gradations with one halftone.
In this case, a production order is provided between a plurality of halftones, so that it is attempted to realize the gradations of the original image with a density represented as an average of the area ratios of the halftones. For example, when the halftones of 141 lines are produced, the production order of 4×4 is provided. As a result, it is possible to realize 304 gradations (4×4×19=304). When the original image has the gradations in a range of 0 to 255, 304 gradations described above are standardized to the output patterns having 256 gradations, so that it is possible to sufficiently realize the gradations of the original image.
In general, the production order between the halftones of the original is systematically determined with an order according to the Bayer arrangement. In the order according to the Bayer arrangement, the halftones are arranged regularly such that the halftones are distributed uniformly in a space. Further, the halftones are arranged so that the arrangement order becomes the highest frequency.
When a pattern has a line number smaller than the line number of the halftones of the original image, the pattern may be looked rough due to the regular production order between the halftones described above. Patent Reference has disclosed a method of reducing the roughness of such a pattern. According to Patent Reference, the production order between the halftones is determined from the standard dot arrangement point according to a blue noise property. It has been mentioned that although the blue noise is a random noise, the blue noise tends to be dispersed in the space uniformly to some extent when a low frequency component of the random noise is eliminated as much as possible. Accordingly, the blue noise tends to be comfortable to human eyes.
As described above, according to Patent Reference, the production order is determined according to the blue noise. Through the process, the regular production order between the halftones is dispersed, so that the resultant pattern becomes uniform in the space to some extent, and has a high frequency component with the low frequency component thus eliminated as much as possible. Accordingly, it is possible to make the pattern with the line number smaller than the line number of the halftones of the original image less outstanding.
Patent Reference: Japanese Patent Publication No. 2002-44445
With the conventional image forming apparatus disclosed in Patent Reference, however, it is difficult to obtain an image with high quality.
In view of the problems described above, an object of the present invention is to provide an image forming apparatus capable of forming an image with high quality.
Further objects and advantages of the invention will be apparent from the following description of the invention.